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Science Forum Index » Medicine - Vision Forum » Question for Dr. Stacy Regarding Floaters
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| Anon E. Muss |
 Posted: Sun Dec 31, 2006 3:31 am |
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On Sat, 30 Dec 2006 12:03:27 -0800, William Stacy <wstacy@obase.net>
wrote:
Quote: Anon E. Muss wrote:
[snip]
Quote: Once again I disagree with the "standard" then.
No use debating with you about it then.
Quote: I consider extraocular motilities to be objective even though people
can fake those too.
Fake a strabismus? Or more important, fake orthophoria when the patient
is a strab?
Not cover/uncover test, but extraocular motilities. You know --
checking for a full range of motion with no gaze restrictions. These
can be "faked".
Quote: P.S. Got a pachymeter about a month ago. :)
A very good objective test, don't you think?
It is (however, the fundus camera we recently got is much more
useful.)
I always recommend measuring central corneal thickness (CCT) for all
ocular hypertensive patients (i.e., <22.5mm Hg per uncorrected
Goldmann applanation tonometry [GAT]) per the OHTS recommendations,
glaucoma suspects (e.g., those with suspicious optic nerves, retinal
nerve fiber layer defects, drance hemorrhages, strong family history,
etc.) and definite glaucoma patients.
Quote: I'll bet you're using it more than you used to "order it" from your
glaucoma guy?
Indeed I am.
BTW, I never ordered it from a "glaucoma guy".
I obtained CCT measurements, objective retinal nerve fiber layer
analysis (i.e., GDx VCC or Stratus OCT), objective optic nerve
analysis (i.e., HRT3) and optic nerve photographs on all my glaucoma
patients/suspects from the special testing clinic at SCCO.
Now that I have a pachymeter and a fundus camera, I do that in house
but still turf out the imaging to SCCO. I do wish they'd get an
Octopus 101 in addition to the HFAs they have there.
BTW, it works out great. Future ODs (students) get exposed to
glaucoma patients, patients get access to cutting edge diagnostic
technology, I know I'm getting the testing I ordered done and my
patients sent back to me versus always wondering with the OMD, I'm
comfortable with the ODs there (for the most part) and I don't have to
worry about expensive lease payments on still evolving technology.
Quote: I had a nice exchange with a lecturer at the Monterey Symposium in
which he asked for a show of hands as to who was "modifying goldmann
readings" by the pachymeter. I was about the only one to raise my
hand. He challenged me and I said I do it in my head, not on paper.
Helps me get a feel of what's going on. He then explained patiently
that my correction tables are not all that accurate. I patiently
explained to him that in most aspects of medicine even variable or
imperfect corrections are better than no corrections at all. Love to
make those guys squirm.
Too bad you weren't at the Academy this year and didn't get the chance
to try and make Murray Fingeret, Harry Quigley and David Friedman
squirm during their glaucoma lecture. Heh.
The Ziemer Ophthalmic rep (maker of the Pascal Dynamic Contour
Tonometry [PDCT]) said they've done a ton of testing comparing GAT to
PDCT in conjunction with CCT. They said that when a cornea is thin,
the GAT usually, but not always, reads low. However, they said that
when a cornea is thick, there was no clear trend -- it could be lower,
higher or unchanged versus PDCT.
And hence the problem with adjusting IOPs based on CCT -- you just
don't know if the adjustments you are making are accurate. Per the
Ziemer rep, a patient with a GAT reading of 19 and a CCT of 595 could
have just as much a chance of having a "real" IOP of 23 as 17. And
the patient with a GAT reading of 17 and a CCT of 515 might have a
"real" IOP of 18 or 23 -- you just don't know. Both Goldmann and
these adjustment algorithms are inprecise and "noisy" and when you
compensate you are IMHO losing more in "noise" than you are gaining in
terms of diagnostic info.
(The "21" or "22.5"mm Hg cutoff for OHTN is arbitrary too -- it's
based on old data that said "21" was 2 standard deviations above the
average. Those numbers don't come from any well-designed studies --
they were literally pulled out of the air.)
One thing we know with a high degree of certainty is that if you are
an OHTN (per uncorrected GAT readings) and have thin corneas, your
risk of developing glaucoma within 5 years is considerably higher than
if you have thick corneas.
Another thing we know is that when one is developing progressive
glaucomatous optic neuropathy, the only proven way to lower the risk
of continued progression is to lower the IOP.
In other words (IOW), it's not so important what a particular person's
exact IOP is (within reason) as far as saying this person does or
doesn't have glaucoma -- more important is if someone has
unquestionably progressive glaucomatous optic neuropathy, their IOP
isn't consistently low enough for their optic nerve (i.e., their optic
nerve is too sick/weak to tolerate that particular IOP) and the IOP
needs to be lowered.
Of course, this "need" needs to be looked at in the context of the
estimated rate of progression, the patient's estimated life span and
the impact on the patient's qualty of life with treatment versus not
treatment. There is a great article by George Spaeth et al (Surv
Ophthalmol. 2006 Jul-Aug;51(4):293-315.) discussing this very issue in
combination with his "Disc Damage Likelihood Scale" he helped develop
(see <http://tinyurl.com/vzhyy>). It's a little complex and
overwhelming for the private practitioner to implement I think, but
the ideas and foundation are quite strong.
In summary, I don't think by compensating for IOPs that you are
obtaining more valuable diagnostic data than I do when I measure CCTs
and merely label those OHTN patients with thin corneas and having an
additional risk factor and those with thick corneas as having a
protective risk factor. And that goes along with the general
consensus of glaucoma experts.
Far better IMHO to go on what the OHTS study said -- thin CCTs are a
strong predictory risk factor in OHTN patients that developed glaucoma
(see <http://tinyurl.com/vlc22>).
Deja vu. I think we've discussed this before. |
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| LarryDoc |
| Posted: Sun Dec 31, 2006 4:56 pm |
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In article <qqpep2ddi9u4gei1eqjgo47b7n6922ivnf@4ax.com>,
Anon E. Muss <anonymous@example.org> wrote:
Quote: In summary, I don't think by compensating for IOPs that you are
obtaining more valuable diagnostic data than I do when I measure CCTs
and merely label those OHTN patients with thin corneas and having an
additional risk factor and those with thick corneas as having a
protective risk factor. And that goes along with the general
consensus of glaucoma experts.
Far better IMHO to go on what the OHTS study said -- thin CCTs are a
strong predictory risk factor in OHTN patients that developed glaucoma
(see <http://tinyurl.com/vlc22>).
Deja vu. I think we've discussed this before.
Indeed. But thanks for taking the time to so concisely present the
issue. This is precisely what the glaucoma specialists in my
neighborhood live by and that which most of us have come to accept as
valid rationale for the diagnosis and management. At least for now.
But on another, possibly related issue:
What of the thousands of LASIK's/PRK'd people with now thin corneas?
Might they have to classified as higher risk glaucoma suspects? Do thin
corneas alone predict risk or is there an underlying genetic cause that
relates simultaneously to thin cornea and glaucoma? Inquiring
minds........
LB, O.D.
(rarely seen around these parts these days) |
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| Anon E. Muss |
| Posted: Sun Dec 31, 2006 10:40 pm |
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On Sun, 31 Dec 2006 20:56:15 GMT, LarryDoc <larrybic@yahoo.remove.com>
wrote:
Quote: But on another, possibly related issue:
What of the thousands of LASIK's/PRK'd people with now thin corneas?
Might they have to classified as higher risk glaucoma suspects?
Not at this time -- there is absolutely no evidence that I am aware of
that iatrogenically thinned corneas (ITC) increase the risk of
developing glaucoma any more than performing full-thickness
penetrating keratoplasty with a "thick" donor graft would be
"glaucomaprotective".
The important thing to realize is that the applanation/indention
tonometry readings you get on ITC will almost certainly be more
inaccurate than ever. And all the "conversion tables" used as far as
I know are based on natural corneal thicknesses, not iatrogenically
altered ones.
Therefore, you really have no idea how much your readings are off
here. I believe it is very fair to assume that your Goldmann
applanation readings (GAT) will be falsely low. How low? It's not
real clear.
So what do I do in situations like this?
[Well, it's like patients I have with tilted discs; in those patients,
I rely a lot more on looking at the fields, the optic nerve
(ONH)/retinal nerve fiber layer (RNFL) imaging and the IOP. Or it's
like the patients I have that are horrible at taking visual fields; in
those patients, I put more weight on the ONH/RNFL imaging, the optic
nerve appearance, the retinal nerve fiber layer and the IOP.]
I really pay more attention to the perimetry results, the imaging
results and the fundus appearance -- realizing that the IOP reading is
not going to be all that accurate, but reasonably assured it will be
precise (see <http://tinyurl.com/y4yglp> under the section "Accuracy
vs precision - the target analogy").
The Ziemer ophthalmic guys say their Pascal Dynamic Contour Tonometer
(PDCT) is great for situations like this -- actually, they say it's
great for everybody and accurate/precise -- but at around $4K a pop
I'm still waiting. I've talked to some people who are commonly
considered to be glaucoma experts in the Optometric community and
their opinions are mixed -- some thing it's useful, others told me
they aren't so sure it's precise.
What the PDCT guys need to remember is that our treatment of glaucoma
is based on many years where GAT was (and still is) the "gold
standard" for IOP measurement. And what we should realize is, and I
wrote about this earlier in the thread, knowing the exact IOP may not
be all that crucial in the vast majority of glaucoma cases. What we
really need is a test that is *precise* (i.e., "off" consistently by
the same amount) from visit to visit, because one of our barometers in
glaucoma treatment is IOP lowering. In other words (IOW), when we
measure IOP, it may not be as important to know whether the IOP really
was "23" before treatment and now it is really "17", rather what is
important is that the knowledge that we *really have* lowered the IOP
by ~25% is reliable. We believe the GAT is precise (enough).
I write this in light of the fact the many people with ocular
hypertension (OHTN) never develop glaucoma and the many people with
IOPs never measured <22mm Hg that develop glaucoma. Precision may be
a lot more important than accuracy, though in a perfect world, we want
a device that is both.
Quote: Do thin corneas alone predict risk or is there an underlying genetic
cause that relates simultaneously to thin cornea and glaucoma?
The general consensus of glaucoma experts I have spoken to and read
articles from believe that central corneal thickness (CCT) is an
indirect measurement of something else that is the real risk factor.
Interesting tidbit:
I didn't know this until the 12/2006 Academy meeting, but Harry
Quigley informed the audience that CCT even being measured in the
Ocular Hypertension Treatment Study (OHTS) was only in there because
one ophthalmologist (OMD) at UC Davis (I don't think he mentioned this
doctor by name, but I believe it is James Brandt) strongly wanted it
in there.
Interestingly, this same OMD -- the one that lobbied to have CCT
measured in the OHTS -- argues *against* adjusting IOPs based on CCT
readings in the 12/01/2006 issue of "Ocular Surgery News" (see
<http://tinyurl.com/y7sq42>) and gives his rationale for such
recommendation -- important quotes:
“Pachymetry over the past few years has come of age with the
recognition that corneal thickness is a very big issue in the
accuracy of measuring IOP and the assessment of risk in our
glaucoma patients,” Dr. Brandt said. “If you have a patient
with a thin cornea and higher pressures, that individual is at
much higher risk of developing glaucoma than somebody with the
same IOP but a thicker cornea.”
and
Because of the inherent error in Goldmann measurement, he
suggested, it is of questionable value to adjust a patient’s
Goldmann IOP by using a nomogram based on corneal thickness.
“You are kidding yourself if you try to apply an algorithm to
adjust by a few millimeters of mercury the underlying
measurement, which is imprecise,” Dr. Brandt said.
Bs pbhefr, guvf jba'g pbaivapr Ovyy Fgnpl :)
P.S. To be fair, there are some that do advocate IOP adjustment based
on CCT such as Leon Herndon in a recent article in "Ophthalmology
Times" (see <http://tinyurl.com/szjy2>), but they are definitely in
the minority. However, in this same article in Ophthalmology Times
James Brandt again counters with a far more (IMHO) compelling
argument. |
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| Anon E. Muss |
| Posted: Sun Dec 31, 2006 10:49 pm |
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On Sun, 31 Dec 2006 18:40:42 -0800, Anon E. Muss
<anonymous@example.org> wrote:
[snip]
Quote: Interestingly, this same OMD -- the one that lobbied to have CCT
measured in the OHTS -- argues *against* adjusting IOPs based on CCT
readings in the 12/01/2006 issue of "Ocular Surgery News" (see
http://tinyurl.com/y7sq42>) and gives his rationale for such
recommendation -- important quotes:
[snip]
Quote: “You are kidding yourself if you try to apply an algorithm to
adjust by a few millimeters of mercury the underlying
measurement, which is imprecise,” Dr. Brandt said.
I believe he meant "inaccurate".  |
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| William Stacy, O.D. |
| Posted: Mon Jan 01, 2007 2:00 pm |
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Anon E. Muss wrote:
Quote:
I write this in light of the fact the many people with ocular
hypertension (OHTN) never develop glaucoma and the many people with
IOPs never measured <22mm Hg that develop glaucoma.
Given the context of your post, I think you meant never measured >22.
w.stacy, o.d. |
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| William Stacy, O.D. |
| Posted: Mon Jan 01, 2007 3:44 pm |
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Anon E. Muss wrote:
Quote: On Mon, 01 Jan 2007 18:00:59 GMT, "William Stacy, O.D."
wstacy@obase.net> wrote:
Anon E. Muss wrote:
I write this in light of the fact the many people with ocular
hypertension (OHTN) never develop glaucoma and the many people with
IOPs never measured <22mm Hg that develop glaucoma.
Given the context of your post, I think you meant never measured >22.
How's this?
A lot of people with consistently "high" IOP never develop glaucoma
and a lot of people with consistently "low" IOP do develop glaucoma.
As a result, IOP measurement is a very poor method of screening
glaucoma patients from normals in the vast majority of the population.
Agreed, it's just that your quote translates: "many people with iops
never measured "LESS THAN 22mm Hg develop glaucoma." That statement is
certainly true, although it would be more meaningful to have said "most
people who never measured less than 22mm Hg have glaucoma".
But what I think you were trying to say was what I originally answered
above.
It's kind of a pet peeve of mine, like when people specify + when they
meant - in an Rx. Or when the order says "DNR" when the patient is not
terminal. Little things like that sometimes are important.
w.stacy, o.d. |
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